Magnetic resonance imaging systems and methods

ABSTRACT

A method is provided for magnetic resonance (MR) imaging near metal, including acquiring an image at a first magnetic field from a subject that includes a metal object, acquiring an image at a second magnetic field, and combining the images to provide a corrected image with reduced metal distortion. An MR imaging system for measuring near metal is also provided including a superconducting magnet to provide a magnetic field, a power supply for a current to ramp the magnetic field, a cryocooler in contact with the superconducting magnet, a magnetic field controller programmed to ramp the main magnetic field by adjusting the current generated by the power supply, a radio frequency system for transmitting and receiving signals, and a data aquisition and processing system to receive the MR signals, generate image data sets and combine the image data sets to provide a corrected image having a reduced metal distortion.

FIELD

The present disclosure relates to systems and methods for magneticresonance imaging (“MRI”). More particularly, the invention relates tosystems and methods for MRI around metal and/or magnets.

BACKGROUND

Standard MRI begins with the assumption that the magnetic fieldexperienced by protons is relatively uniform over the imaging volume.The presence of materials with different magnetic susceptibilities cancause inhomogeneities in the field experienced by protons over theimaging volume. In particular, materials with large magneticsusceptibility constants, such as metals or magnetic materials, cancause very large inhomogeneities, making it difficult or impossible toimage protons in neighbourhoods around these substances.

To try to perform MR imaging around metal objects, multi-spectralmethods have been proposed and implemented. Advanced imaging aroundmetal methods, such as multi-acquisition with variable resonance imagecombination (MAVRIC), slice-encoding metal artifact correction (SEMAC)and hybrid approaches, attempt to recover signal from areas around metalby exciting and receiving signal at different frequencies by offsettingthe transmit/receive frequency of the resonant frequency (RF) chain.This “tuning” to different resonant frequencies is done by transmittingand receiving with the resonant frequency chain at different narrowbands around different offset frequencies. However resonant frequencychains have a limited operating bandwidth over which they remain tunedand capable of operating (typically less than or equal to 1 MHz), whichlimits the range of frequencies that can be explored. This limits thesemethods to only probing frequencies within the tuned range of the RFtransmit and receive chain.

SUMMARY

A further understanding of the functional and advantageous aspects ofthe invention can be realized by reference to the following detaileddescription and drawings.

An object of the present disclosure is to provide a system and methodfor MRI around metal or magnetic objects.

Thus by one broad aspect of the present invention, a method for MRI isprovided including acquiring a first image data from a subject with ametal object using an MRI system having a main magnetic field at a firstmagnetic field strength and using a first transmit frequency, adjustingthe main magnetic field of the MRI system to a second magnetic fieldstrength, acquiring a second image data from the subject using the MRIsystem while the main magnetic field of the MR imaging system is at thesecond magnetic field strength, and combining the first and second imagedata to provide a corrected image having a reduced metal distortion.

By a further broad aspect of the present invention, a system is providedincluding a superconducting magnet for generating a main magnetic field,a power supply for providing a current for ramping the main magneticfield, a switch selectively connecting the superconducting magnet to thepower supply and having an open state and a closed state, wherein whenin the closed state the switch connects the superconducting magnet andthe power supply in a connected circuit, a mechanical cryocooler inthermal contact with the superconducting magnet and operable to reduceand maintain a temperature of the superconducting magnet below atransition temperature of the superconducting magnet, a magnetic fieldcontroller programmed to ramp the main magnetic field from a firstmagnetic field strength to a second magnetic field strength by setting acurrent generated by the power supply to an initial current value,activating the switch to its closed position, thereby connecting thesuperconducting magnet and the power supply in the connected circuit,adjusting the current generated by the power supply, and activating theswitch to its open position when the second magnetic field strength isreached, thereby disconnecting the superconducting magnet and the powersupply from the connected circuit and placing the superconducting magnetin a closed circuit, a gradient system positioned about a bore of thesuperconducting magnet for producing magnetic field gradients, a radiofrequency (RF) system for transmitting RF excitation signals andreceiving MR signals, a data acquisition system connected to the RFsystem to receive the MR signals from the RF system, a data processingsystem configured to receive the MR signals from the data acquisitionsystem, sort the MR signals into a plurality of MR data sets, each MRdata set acquired at a distinct magnetic field strength, generate aplurality of image data sets corresponding to the plurality of MR datasets, and combine the plurality of image data sets to provide acorrected image having a reduced metal distortion, a pulse controllerconnected to the RF system, gradient system and data acquisition systemthat generates pulse sequences that include RF pulses from the RF systemand gradient pulses from the gradient system, and a computer storage forstoring the corrected image.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments disclosed herein will be more fully understood from thefollowing detailed description taken in connection with the accompanyingdrawings, which form a part of this application, and in which:

FIG. 1 is a diagram illustrating contour plots of an object withmagnetic susceptibility in a magnetic field.

FIG. 2 is a diagram illustrating contour plots of the magnetic fieldvariation around a metal implant.

FIG. 3 is a diagram illustrating plots indicating regions in a medicalresonance image where tissue can be imaged in the presence of a metallicimplant with a susceptibility of approximately 150 ppm.

FIG. 4 is a diagram illustrating plots indicating regions in a medicalresonance image where tissue can be imaged in the presence of a metallicimplant with a susceptibility of approximately 6000 ppm.

FIG. 5 is a flowchart setting forth the steps of an example method formagnetic field-dependent multi-spectral imaging using an MRI system andgenerating integrated images based on MRI signal data.

FIG. 6 is a block diagram of an example MRI system that can implementthe method of the present invention.

DETAILED DESCRIPTION

Various embodiments and aspects of the disclosure will be described withreference to details discussed below. The following description anddrawings are illustrative of the disclosure and are not to be construedas limiting the disclosure. Numerous specific details are described toprovide a thorough understanding of various embodiments of the presentdisclosure. However, in certain instances, well-known or conventionaldetails are not described in order to provide a concise discussion ofembodiments of the present disclosure.

Various apparatuses or processes will be described below to provideexamples of embodiments of the imaging method and system disclosedherein. No embodiment described below limits any claimed embodiment andany claimed embodiments may cover processes or apparatuses that differfrom those described below. The claimed embodiments are not limited toapparatuses or processes having all of the features of any one apparatusor process described below or to features common to multiple or all ofthe apparatuses or processes described below. It is possible that anapparatus or process described below is not an embodiment of any claimedinvention.

Furthermore, numerous specific details are set forth in order to providea thorough understanding of the embodiments described herein. However,it will be understood by those of ordinary skill in the art that theembodiments described herein may be practiced without these specificdetails. In other instances, well-known methods, procedures andcomponents have not been described in detail so as not to obscure theembodiments described herein.

Unless defined otherwise, all technical and scientific terms used hereinare intended to have the same meaning as commonly understood to one ofordinary skill in the art. Unless otherwise indicated, such as throughcontext, as used herein, the following terms are intended to have thefollowing meanings:

Normal MRI relies on the magnetic field experienced over the imagingvolume to be homogeneous. MR methods of spatial encoding (sliceselection, phase encoding, frequency encoding) are then applied byperturbing the magnetic field as a function of spatial position in knownways such that the signal from different positions has well-definedcharacteristics. These encoding methods impose spatially varyingmagnetic field changes that are usually on the order of <1 kHz per imagevoxel.

The proton signal that is normally acquired in MR precesses at aresonant frequency (RF) that is determined by the magnetic fieldexperienced by protons at each spatial location. Normally this resonantfrequency is just a function of the main magnetic field (Ho). In thepresence of metal objects however, the effective magnetic fieldexperienced by protons becomes a function of the main magnetic fieldplus the magnetic field perturbation generated by the metal object, ie.H_(eff)=H_(o)+H_(metal) where H_(metal) is a function of spatialposition, the magnetic susceptibility of the metal, and can itselfdepend on the main magnetic field strength.

Referring to FIG. 1, contour plots are illustrated for an object 110with some magnetic susceptibility property within different mainmagnetic field strengths. Examples of field lines 120 logarithmicallyspaced between −1 to 1 MHz field variation are shown which arerepresentative of the effects for, from top to bottom, a magnetic fieldof 0.5 T, 1.5 T and 3.0 T. The effective magnetic field experienced by aproton in the vicinity of the object 110 is dependent not only on themagnetic susceptibility of the object and the proton's position relativeto the object, but also on the strength of the main magnetic field.

To try to perform MR imaging around metal objects, existing advancedmulti-spectral methods have been proposed and implemented. Advancedimaging around metal methods (such as MAVRIC and SEMAC and hybridapproaches), attempt to recover signal from areas around metal byexciting and receiving signal at different frequencies by offsetting thetransmit/receive frequency of the RF chain. Multispectral methods arebased on the fact that protons in neighbourhoods of metal objects have aresonant frequency given by H_(eff) (not just H_(o)), so by repeating animaging experiment with the experiment “tuned” to different resonantfrequencies, one can obtain information from protons that areexperiencing different H_(eff) fields, i.e. at different spatialpositions in the neighbourhood of metal objects. This “tuning” todifferent resonant frequencies is done by transmitting and receivingwith the resonant frequency chain at different narrow bands arounddifferent offset frequencies. However resonant frequency chains have alimited operating bandwidth over which they remain tuned and capable ofoperating (typically less than or equal to 1 MHz). This limits thesemethods to only probing frequencies within the tuned range of the RFtransmit and receive chain (typically ˜1 MHz).

The systems and methods of the present invention utilize an MRI systemthat can be operated to rapidly ramp its main magnetic field strength,thereby allowing for measurements at multiple different field strengths.The main magnetic field is changed using a dynamically rampable magnetinstead of, or in conjunction with, changing the transmit/receive offsetfrequency of the resonant frequency chain. By ramping the main magnet todifferent field strengths in conjunction with transmit/receive offsetfrequencies within the 100's of Hz of optimal tuning range of the RFsubsystem, a much larger range of frequencies can be explored. Changingthe main magnetic field can allow one to explore a wider range ofresonant frequencies, and thus explore protons which are experiencing awider range of H_(eff) fields. Thus, changing the applied main magneticfield may allow imaging of protons which normally have resonantfrequencies outside the range of what can be probed with offsetfrequencies alone. For example, using a rampable magnet to change themain magnetic field may enable imaging closer to metal objects thanexisting multi-spectral approaches allow. Using a rampable magnet tochange the main magnetic field may also enable imaging in theneighbourhood of metal objects with higher magnetic susceptibilityconstants than can currently be imaged (e.g. near magnetic materials).

For example, at 1.5 T the water proton resonant frequency is:

GAMMA*1.5T=63.855 MHz (GAMMA=42.57 MHz/T for hydrogen)

For a material with a magnetic susceptibility of X, a normal systemcould still detect it if the material distorts the field at/inside thatmaterial no more than 0.5 MHz off from the normal resonance. i.e. for

GAMMA*(1+X)*1.5T=(63.855+0.5) MHz

which implies, the limit is for materials with a magnetic susceptibilitythat is approx. X=0.0078 (ie. 0.0078 times different than water).

By ramping the magnetic field down to 0.5 T, the normally tuned RFsubsystem would be able to detect protons at/in materials whose magneticsusceptibility satisfies:

GAMMA*(1+X)*0.5T=63.855 MHz

which implies the limit now is for materials with a magneticsusceptibility as high as X=2.0.

Referring now to FIG. 2, contour plots of the magnetic field variationaround a metal implant are illustrated. The left panel in FIG. 2illustrates simulated contour plots 210 around a metal 220 withsusceptibility approximately 150 ppm (e.g. titanium implant). The rightpanel of FIG. 2 illustrates simulated contour plots 230 around a metal240 with susceptibility approximately 6000 ppm (e.g. non-magneticstainless steel). A solid line contour 250, 260 represents a 10 kHzvariation from the background (main) magnetic field. Contours on bothplots are shown with dotted lines 210, 230 for field offsets of 100 Hz,500 Hz, 1 kHz, 2.5 khz, 5 kHz, 10 kHz, 25 kHz, 50 kHz, 100 kHz and 300kHz.

FIG. 3 illustrates plots indicating regions in a medical resonance imagewhere tissue can be successfully imaged (grey shaded regions) in thepresence of a metallic implant (black bar) 310 with a susceptibility ofapproximately 150 ppm. The top image shows the region 320 that can beimaged using typical on-resonance RF excitation pulses (e.g. regionswith less than 1 kHz field offset). The middle row images indicateregions 330, 340, 350, 360, 370 that can be imaged using RF excitationpulses offset to frequency bands representative of normal multispectralmethods used to image around metal. Frequency bands are, from left toright, 1-2 kHz, 2-4 kHz, 4-6 kHz, 6-8 kHz, 8-10 kHz. The bottom imageindicates the region 380 that can be expected to be effectively imagedusing existing multispectral (imaging around metal) acquisitions.

Referring now to FIG. 4, plots indicating regions in an image wheretissue can be successfully imaged (grey shaded regions) in the presenceof a metallic implant (black bar) 410 with susceptibility ofapproximately 6000 ppm are illustrated. The top image shows the region420 that can be imaged using typical on-resonance RF excitation pulses(e.g. regions with less than 1 kHz field offset). The middle row imagesindicate regions 430, 440, 450, 460, 470 that can be imaged using RFexcitation pulses offset to excite different off-resonance frequencybands. The left-most image represents the region that is typically ableto be imaged with existing multispectral methods. The remaining imagesrepresent frequency bands at much greater off-resonance bands, namely10-50 kHz, 50-100 kHz, 100-200 kHz and 200-300 kHz. Many of these offsetfrequencies can be more easily accessible by changing the system mainmagnetic field and using on-resonance RF excitation pulses to ensurepeak RF sensitivity and performance. The bottom left image indicates theregion 480 that can be expected to be effectively imaged using existingmultispectral methods. The bottom right image indicates the region 490that can be expected to be effectively imaged using broader frequencyoffset bands that are accessible by changing the main magnetic fieldduring the acquisition of multispectral data.

Using multiple magnetic field strengths provides access to a wider rangeof offset resonant frequencies. It also provides different magneticfield environments to the metal/magnetic material, that may result indifferent magnetic behavior or H_(metal) of the material. The differentresponses of the material may identify an optimal magnetic fieldstrength at which to produce an artifact-reduced image.

Referring now to FIG. 5, a flowchart is illustrated setting forth thesteps of an example method for imaging near metal objects and/or magnetsby changing the main magnetic field with or without using multispectralapproaches.

The method includes directing the MRI system to perform a pulse sequencethat acquires data by sampling a magnetic resonance signal at varioustime points, as indicated at step 502. In general, the magneticresonance signal is generated by nuclear spins relaxing back toequilibrium and thus can include a free induction decay (“FID”) signal,a gradient echo signal, a spin echo signal, a stimulated echo signal, orany other suitable magnetic resonance signal. In general, the MRI systemis operated to generate magnetic resonance signals across a region ofthe subject, such as an image slice, image slab, image volume, or otherspatially localized region-of-interest.

It will be appreciated by those skilled in the art that the choice ofpulse sequence will influence the type of magnetic resonance signal thatis formed and also the relaxation parameter to be studied. For instance,an inversion recovery or T1-weighted pulse sequence may be used forexamining longitudinal relaxation, whereas a T2-weighted pulse sequencemay be used for examining transverse relaxation.

A determination is made at decision block 504 whether MR data have beenacquired at a desired number of frequency bands. If not, the excitationfrequency and reception frequency are adjusted, as indicated at step 506and additional data are acquired at the new offset frequency. Forexample, data may be collected using on-resonance RF excitation pulses,followed by data collection using offset RF excitation pulses to excitedifferent off-resonance frequency bands.

When data have been acquired at all of the desired resonant frequencies,a further determination is made at decision block 508 whether data havebeen acquired at a desired number of different magnetic field strengths.If not, then the magnetic field strength is adjusted, as indicated atstep 510, and additional data are acquired at the new magnetic fieldstrength. Thus, the method includes acquiring data at least at a firstmagnetic field strength and a second magnetic field strength; however,the process can be generally repeated to acquire a plurality of datasets at each of a plurality of different magnetic field strengths.Accordingly, in an MRI system, a tunable RF coil may be implemented,such that the RF coil can be tuned to the appropriate resonancefrequencies associated with the different magnetic field strengths.Likewise, a broadband receiver can be implemented to provide a widerange of resonance frequencies, and thus field strengths, that can beused in a single scan. Examples of such tunable RF coils and broadbandreceivers will be appreciated by those skilled in the art.

In another embodiment, a single transmit/receive frequency and bandwidthfor each imaging slice is used and the multi-spectral sampling is donesolely by changing the main magnetic field strength. This is useful forexample where the magnetic material is saturated. A tunable RF system isnot needed if the material is saturated such that the magnetic field itproduces (H_(metal)) does not depend on the background magnetic field(H_(o)). In this case, the excitation bands move as the additive field(H_(eff)=background field+material field) moves as the background fieldchanges, but a single transmit/receive frequency and bandwidth is used.This would be the case when imaging around highly magnetic materialssuch as magnetic stainless steel.

In one specific embodiment, the magnetic field is adjusted using an MRIsystem that is capable of rapidly ramping up or down its main magneticfield. An example of such a system is described in WO2017/064539 A1,“MAGNETIC RESONANCE IMAGING SYSTEM CAPABLE OF RAPID FIELD RAMPING”, theentirety of which is incorporated herein. With this type of system, themain magnetic field can be ramped to different strengths depending onthe amount of applied current. Advantageously, the main magnetic fieldcan be ramped in a clinically reasonable amount of time. As one example,the main magnetic field can be ramped from zero to 0.5 Tin about tenminutes. As another example, the main magnetic field can be ramped from0.5 T to 0.4 T in about one minute or less. Thus, in some embodiments,the magnetic field strength of the main magnetic field can be rampedbetween a first and second value in less than fifteen minutes, and insome embodiments less than two minutes.

Using such a system, then, the main magnetic field can be incrementallyadjusted to acquire data at multiple different field strengths in aclinically reasonable span of time, thereby providing magneticfield-dependent relaxometry. As one example, the main magnetic field canbe adjusted in increments of 0.1 T; however, it will be appreciated thatother increments greater or less than 0.1 T can also be used (e.g., 0.05T, 0.2 T, 0.25 T, 0.5 T). In some embodiments, the main magnetic fieldcan be further adjusted using a coil insert to modulate the localmagnetic field in the MRI system.

When data have been acquired at all of the desired resonant frequenciesfor each of the desired magnetic field strengths, the acquired data arecombined as indicated at step 512. In one embodiment, independentspectral bin images are combined using quadrature summation to form acomposite image for the data acquired at each magnetic field strength.Composite images from each magnetic field strength are then combined toprovide an integrated image having reduced artifacts produced by themetal or magnet.

In a further embodiment, a frequency image is reconstructed for each MRdata set or spectral bin acquired, resulting in a collection offrequency images. To minimize the metal or magnet-induced distortion,the pixels for each of the plurality of MR frequency images may beexamined to determine, for each pixel location, which image of theplurality of MR images has the maximum intensity. Each pixel location isassigned the maximum intensity for the corresponding pixel location andthat value is used. The frequency images are converted or transformedinto temporal images. The plurality of temporal images is used togenerate a combined image with reduced metal/magnet-induced artifacts.

In another embodiment, a frequency image is reconstructed for each MRdata set or spectral bin acquired at a single magnetic field strength,resulting in a collection of frequency images. To minimize the metal ormagnet-induced distortion, the pixels for each of the plurality of MRfrequency images may be examined to determine, for each pixel location,which image of the plurality of MR images has the maximum intensity.Each pixel location is assigned the maximum intensity for thecorresponding pixel location and that value is used. The frequencyimages are converted or transformed into temporal images. A combinedimage for each magnetic field strength is generated based on theplurality of temporal images, and the combined images generated for eachmagnetic field strength are further combined to generate an image withreduced artfacts.

As indicated at step 514, the images are stored in computer memory andat step 516 may be displayed to a user on a computer display.

Using multiple magnetic field strengths provides access to a wider rangeof offset resonant frequencies. It also provides different magneticstrength environments to the metal/magnet material, that may result invarying magnetic susceptibility of the material. The different responsesof the material may identify an optimal magnetic field strength at whichto produce an artifact-reduced image for that particular material.

Example MRI System

Referring now to FIG. 6, an example of an MRI system 610 that is capableof rapidly ramping its magnetic field is illustrated. The MRI system 610generally includes a magnet assembly 612 for providing a magnetic field614 that is substantially uniform within a bore 616 that may hold asubject 618 or other object to be imaged. The magnet assembly 612supports a radio frequency (“RF”) coil (not shown) that may provide anRF excitation to nuclear spins in the subject 618 or object positionedwithin the bore 616. The RF coil communicates with an RF system 620producing the necessary electrical waveforms, as is understood in theart. The RF coil can be a tunable RF coil that can be tuned to variousdifferent resonance frequencies (e.g., resonance frequencies associatedwith different magnetic field strengths), as is understood in the art.The RF system 620 can include a broadband receiver capable of receivingmagnetic resonance signals across a broad range of resonancefrequencies, thereby allowing a similarly broad range of differentmagnetic field strengths to be implemented.

The magnet assembly 612 also supports three axes of gradient coils (notshown) of a type known in the art, and which communicate with acorresponding gradient system 622 providing electrical power to thegradient coils to produce magnetic field gradients, G_(x), G_(y), andG_(z) over time.

A data acquisition system 624 connects to RF reception coils (not shown)that are supported within the magnet assembly 612 or positioned withinbore 616.

The RF system 620, gradient system 622, and data acquisition system 624each communicates with a controller 626 that generates pulse sequencesthat include RF pulses from the RF system 620 and gradient pulses fromgradient system 622. The data acquisition system 624 receives magneticresonance signals from the RF system 620 and provides the magneticresonance signals to a data processing system 628, which operates toprocess the magnetic resonance signals and to reconstruct imagestherefrom. The reconstructed images can be provided to a display 630 fordisplay to a user.

The magnet assembly 612 includes one or more magnet coils 632 housed ina vacuum housing 634, which generally provides a cryostat for the magnetcoils 632, and mechanically cooled by a mechanical cryocooler 636, suchas a Gifford-McMahon (“GM”) cryocooler or a pulse tube cryocooler. Inone example configuration, the cryocooler can be a Model RDK-305Gifford-McMahon cryocooler manufactured by Sumitomo Heavy Industries(Japan). In general, the cryocooler 636 is in thermal contact with themagnet coils 632 and is operable to lower the temperature of the magnetcoils 632 and to maintain the magnet coils 632 and a desired operatingtemperature. In some embodiments the cryocooler 636 includes a firststage in thermal contact with the vacuum housing 634 and a second stagein thermal contact with the magnet coils 632. In these embodiments, thefirst stage of the cryocooler 636 maintains the vacuum housing 634 at afirst temperature and the second stage of the cryocooler 636 maintainsthe magnet coils 632 at a second temperature that is lower than thefirst temperature.

The magnet coils 632 are composed of a superconducting material andtherefore provide a superconducting magnet. The superconducting materialis preferably selected to be a material with a suitable criticaltemperature such that the magnet coils 632 are capable of achievingdesired magnetic field strengths over a range of suitable temperatures.As one example, the superconducting material can be niobium (“Nb”),which has a transition temperature of about 9.2 K. As another example,the superconducting material can be niobium-titanium (“NbTi”), which hasa transition temperature of about 10 K. As still another example, thesuperconducting material can be triniobium-tin (“Nb₃Sn”), which has atransition temperature of about 18.3 K.

The choice of superconducting material will define the range of magneticfield strengths achievable with the magnet assembly 612. Preferably, thesuperconducting material is chosen such that magnetic field strengths inthe range of about 0.0 T to about 3.0 T can be achieved over a range oftemperatures that can be suitably achieved by the cryocooler 636. Insome configurations, however, the superconducting material can be chosento provide magnetic field strengths higher than 3.0 T.

The cryocooler 636 is operable to maintain the magnet coils 632 at anoperational temperature at which the magnet coils 632 aresuperconducting, such as a temperature that is below the transition, orcritical, temperature for the material of which the magnet coils 632 arecomposed. As one example, a lower operational temperature limit can beabout 4 K and an upper operational temperature limit can be at or nearthe transition, or critical, temperature of the superconducting materialof which the magnet coils 632 are composed.

The current density in the magnet coils 632 in the MRI system 610 iscontrollable to rapidly ramp up or ramp down the magnetic field 614generated by the magnet assembly 612 while controlling the temperatureof the magnet coils 632 with the cryocooler 636 to keep the temperaturebelow the transition temperature of the superconducting material ofwhich the magnet coils 632 are composed. As one example, the magneticfield 614 can be ramped up or ramped down on the order of minutes, suchas fifteen minutes or less.

In general, the current density in the magnet coils 632 can be increasedor decreased by connecting the magnet coils 632 to a circuit with apower supply 638 that is in electrical communication with the magnetcoils 632 via a switch 640 and operating the power supply 638 toincrease or decrease the current in the connected circuit. The switch640 is generally a superconducting switch that is operable between afirst, closed, state and a second, open, state.

When the switch 640 is in its open state, the magnet coils 632 are in aclosed circuit, which is sometimes referred to as a “persistent mode.”In this configuration, the magnet coils 632 are in a superconductingstate so long as the temperature of the magnet coils 632 is maintainedat a temperature at or below the transition temperature of thesuperconducting material of which they are composed.

When the switch 640 is in the closed state, however, the magnet coils632 and the power supply 638 can be placed in a connected circuit, andthe current supplied by the power supply 638 and the current in themagnet coils 632 will try to equalize. For instance, if the power supply638 is operated to supply more current to the connected circuit, thecurrent in the magnet coils 632 will increase, which will increase thestrength of the magnetic field 614. On the other hand, if the powersupply 638 is operated to decrease the current in the connected circuit,the current in the magnet coils 632 will decrease, which will decreasethe strength of the magnetic field 614.

It will be appreciated by those skilled in the art that any suitablesuperconducting switch can be used for selectively connecting the magnetcoils 632 and power supply 638 into a connected circuit; however, as onenon-limiting example, the switch 640 may include a length ofsuperconducting wire that is connected in parallel to the magnet coils632 and the power supply 638. To operate such a switch 640 into itsclosed state, a heater in thermal contact with the switch 640 isoperated to raise the temperature of the superconducting wire above itstransition temperature, which in turn makes the wire highly resistivecompared to the inductive impedance of the magnet coils 632. As aresult, very little current will flow through the switch 640. The powersupply 638 can then be placed into a connected circuit with the magnetcoils 632. When in this connected circuit, the current in the powersupply 638 and the magnet coils 632 will try to equalize; thus, byadjusting the current supplied by the power supply 638, the currentdensity in the magnet coils 632 can be increased or decreased torespectively ramp up or ramp down the magnetic field 614. To operate theswitch 640 into its open state, the superconducting wire in the switch640 is cooled below its transition temperature, which places the magnetcoils 632 back into a closed circuit, thereby disconnecting the powersupply 638 and allowing all of the current to flow through the magnetcoils 632.

When the magnet coils 632 are in the connected circuit with the powersupply 638, the temperature of the magnet coils 632 will increase as thecurrent in the connected circuit equalizes. Thus, the temperature of themagnet coils 632 should be monitored to ensure that the temperature ofthe magnet coils 632 remains below the transition temperature for thesuperconducting material of which they are composed. Because placing themagnet coils 632 into a connected circuit with the power supply 638 willtend to increase the temperature of the magnet coils 632, the rate atwhich the magnetic field 614 can be ramped up or ramped down will dependin part on the cooling capacity of the cryocooler 636. For instance, acryocooler with a larger cooling capacity will be able to more rapidlyremove heat from the magnet coils 632 while they are in a connectedcircuit with the power supply 638.

The power supply 638 and the switch 640 operate under control from thecontroller 626 to provide current to the magnet coils 632 when the powersupply 638 is in a connected circuit with the magnet coils 632. Acurrent monitor 642 measures the current flowing to the magnet coils 632from the power supply 638, and a measure of the current can be providedto the controller 626 to control the ramping up or ramping down of themagnetic field 614. In some configurations, the current monitor 642 isintegrated into the power supply 638.

A temperature monitor 644 is in thermal contact with the magnet assembly612 and operates to measure a temperature of the magnet coils 632 inreal-time. As one example, the temperature monitor 644 can include athermocouple temperature sensor, a diode temperature sensor (e.g., asilicon diode or a GaAlAs diode), a resistance temperature detector(“RTD”), a capacitive temperature sensor, and so on. RTD-basedtemperature sensors can be composed of ceramic oxynitride, germanium, orruthenium oxide. The temperature of the magnet coils 632 is monitoredand can be provided to the controller 626 to control the ramping up orramping down of the magnetic field 614.

In operation, the controller 626 is programmed to ramp up or ramp downthe magnetic field 614 of the magnet assembly 612 in response toinstructions from a user. As mentioned above, the magnetic field 614 canbe ramped down by decreasing the current density in the magnet coils 632by supplying current to the magnet coils 632 from the power supply 638via the switch 640, which is controlled by the controller 626. Likewise,the strength of the magnetic field 614 can be ramped up by increasingthe current density in the magnet coils 632 by supplying current to themagnet coils 632 from the power supply 638 via the switch 640, which iscontrolled by the controller 626.

The controller 626 is also programmed to monitor various operationalparameter values associated with the MRI system 610 before, during, andafter ramping the magnetic field 614 up or down. As one example, asmentioned above, the controller 626 can monitor the current supplied tothe magnet coils 632 by the power supply 638 via data received from thecurrent monitor 642. As another example, as mentioned above, thecontroller 626 can monitor the temperature of the magnet coils 632 viadata received from the temperature monitor 644. As still anotherexample, the controller 626 can monitor the strength of the magneticfield 614, such as by receiving data from a magnetic field sensor, suchas a Hall probe or the like, positioned in or proximate to the bore 616of the magnet assembly 612.

One or more computer systems can be provided with the MRI system 610 forprocessing acquired data in accordance with the methods described above.As one example, the data processing system 628 can be used to processthe acquired data.

For example, the data processing system 628 can receive magneticresonance data from the data acquisition system 624 and process it inaccordance with instructions downloaded from an operator workstation.Such processing may include those methods described above forreconstructing images, fitting signals to signal models, generatingdispersion data, and computing quantitative or physical parameters fromdispersion data.

Images reconstructed by the data processing system 628 can be conveyedback to the operator workstation for storage, and real-time images canbe stored in a memory, from which they may be output to display 630.

The MRI system 610 may also include one or more networked workstations.By way of example, a networked workstation may include a display; one ormore input devices, such as a keyboard and mouse; and a processor. Thenetworked workstation may be located within the same facility as the MRIsystem 610, or in a different facility, such as a different healthcareinstitution or clinic.

The networked workstation, whether within the same facility or in adifferent facility as the MRI system 610, may gain remote access to thedata processing system 628 via a communication system. Accordingly,multiple networked workstations may have access to the data processingsystem 628. In this manner, magnetic resonance data, reconstructedimages, or other data may be exchanged between the data processingsystem 628 and the networked workstations, such that the data or imagesmay be remotely processed by a networked workstation. This data may beexchanged in any suitable format, such as in accordance with thetransmission control protocol (“TCP”), the internet protocol (“IP”), orother known or suitable protocols.

Generally, a computer, computer system, computing device, client orserver, as will be well understood by a person skilled in the art,includes one or more than one electronic computer processor, and mayinclude separate memory, and one or more input and/or output (I/O)devices (or peripherals) that are in electronic communication with theone or more processor(s). The electronic communication may befacilitated by, for example, one or more busses, or other wired orwireless connections. In the case of multiple processors, the processorsmay be tightly coupled, e.g. by high-speed busses, or loosely coupled,e.g. by being connected by a wide-area network.

A computer processor, or just “processor”, is a hardware device forperforming digital computations. It is the express intent of theinventors that a “processor” does not include a human; rather it islimited to be an electronic device, or devices, that perform digitalcomputations. A programmable processor is adapted to execute software,which is typically stored in a computer-readable memory. Processors aregenerally semiconductor based microprocessors, in the form of microchipsor chip sets. Processors may alternatively be completely implemented inhardware, with hard-wired functionality, or in a hybrid device, such asfield-programmable gate arrays or programmable logic arrays. Processorsmay be general-purpose or special-purpose off-the-shelf commercialproducts, or customized application-specific integrated circuits(ASICs). Unless otherwise stated, or required in the context, anyreference to software running on a programmable processor shall beunderstood to include purpose-built hardware that implements all thestated software functions completely in hardware.

Multiple computers (also referred to as computer systems, computingdevices, clients and servers) may be networked via a computer network,which may also be referred to as an electronic network or an electroniccommunications network. When they are relatively close together thenetwork may be a local area network (LAN), for example, using Ethernet.When they are remotely located, the network may be a wide area network(WAN), such as the internet, that computers may connect to via a modem,or they may connect to through a LAN that they are directly connectedto.

Computer-readable memory, which may also be referred to as acomputer-readable medium or a computer-readable storage medium, whichterms have identical (equivalent) meanings herein, can include any oneor a combination of non-transitory, tangible memory elements, such asrandom access memory (RAM), which may be DRAM, SRAM, SDRAM, etc., andnonvolatile memory elements, such as a ROM, PROM, FPROM, OTP NVM, EPROM,EEPROM, hard disk drive, solid state disk, magnetic tape, CDROM, DVD,etc.) Memory may employ electronic, magnetic, optical, and/or othertechnologies, but excludes transitory propagating signals so that allreferences to computer-readable memory exclude transitory propagatingsignals. Memory may be distributed such that at least two components areremote from one another, but are still all accessible by one or moreprocessors. A nonvolatile computer-readable memory refers to acomputer-readable memory (and equivalent terms) that can retaininformation stored in the memory when it is not powered. Acomputer-readable memory is a physical, tangible object that is acomposition of matter. The storage of data, which may be computerinstructions, or software, in a computer-readable memory physicallytransforms that computer-readable memory by physically modifying it tostore the data or software that can later be read and used to cause aprocessor to perform the functions specified by the software or tootherwise make the data available for use by the processor. In the caseof software, the executable instructions are thereby tangibly embodiedon the computer-readable memory. It is the express intent of theinventor that in any claim to a computer-readable memory, thecomputer-readable memory, being a physical object that has beentransformed to record the elements recited as being stored thereon, isan essential element of the claim.

Software may include one or more separate computer programs configuredto provide a sequence, or a plurality of sequences, of instructions toone or more processors to cause the processors to perform computations,control other devices, receive input, send output, etc.

It is intended that the invention includes computer-readable memorycontaining any or all of the software described herein. In particular,the invention includes such software stored on non-volatilecomputer-readable memory that may be used to distribute or sellembodiments of the invention or parts thereof.

While the applicant's teachings described herein are in conjunction withvarious embodiments for illustrative purposes, it is not intended thatthe applicant's teachings be limited to such embodiments. On thecontrary, the applicant's teachings described and illustrated hereinencompass various alternatives, modifications, and equivalents, withoutdeparting from the embodiments, the general scope of which is defined inthe appended claims. Except to the extent necessary or inherent in theprocesses themselves, no particular order to steps or stages of methodsor processes described in this disclosure is intended or implied. Inmany cases the order of process steps may be varied without changing thepurpose, effect, or import of the methods described.

1. A method for magnetic resonance (MR) imaging near metal comprising:acquiring a first image data from a subject including a metal objectusing an MR imaging system having a main magnetic field at a firstmagnetic field strength and using a first transmit frequency; adjustingthe main magnetic field of the MRI system to a second magnetic fieldstrength; acquiring a second image data from the subject using the MRimaging system while the main magnetic field of the MR imaging system isat the second magnetic field strength; and combining the first andsecond image data to provide a corrected image having a reduced metaldistortion, wherein the second magnetic field strength is lower than thefirst magnetic field strength; the second transmit frequency comprises aresonant frequency and at least one offset frequency; and the number ofoffset frequencies of the first transmit frequency is fewer than thenumber of offset frequencies of the second transmit frequency.
 2. Themethod for MR imaging near metal as in claim 1, further comprising usinga second transmit frequency while the main magnetic field of the MRimaging system is at the second magnetic field strength.
 3. The methodfor MR imaging near metal as in claim 1, wherein the first transmitfrequency comprises a resonant frequency and at least one offsetfrequency, and the first image data is a sum of the image data from theresonant frequency and the at least one offset frequency.
 4. The methodfor MR imaging near metal as in claim 2, wherein the second transmitfrequency comprises a resonant frequency and at least one offsetfrequency, and the second image data is a sum of the image data from theresonant frequency and the at least one offset frequency.
 5. (canceled)6. The method for MR imaging near metal as in claim 1, wherein the firstmagnetic field strength and the second magnetic field strength areselected based on a material of a metal implant.
 7. The method for MRimaging near metal as in claim 1, wherein the magnetic field is adjustedto the second magnetic field strength by: selecting a ramp functiondefining at least one ramp rate; setting a current generated by a powersupply to an initial current value; activating a superconducting switchto its closed position, thereby connecting a superconducting magnet ofthe MR imaging system and the power supply in a connected circuit;adjusting the current generated by the power supply according to theselected ramp function; and activating the superconducting switch to itsopen position when the second magnetic field strength is reached,thereby disconnecting the superconducting magnet and the power supplyfrom the connected circuit and placing the superconducting magnet in aclosed circuit.
 8. The method for MR imaging near metal as in claim 7,wherein selecting a ramp function comprises selecting a short ramp timefor adjusting the magnetic field to the second magnetic field strength.9. A magnetic resonance (MR) imaging system for measuring near metalcomprising: a superconducting magnet for generating a main magneticfield; a power supply for providing a current for ramping the mainmagnetic field; a switch selectively connecting the superconductingmagnet to the power supply and having an open state and a closed state,wherein when in the closed state the switch connects the superconductingmagnet and the power supply in a connected circuit; a mechanicalcryocooler in thermal contact with the superconducting magnet andoperable to reduce and maintain a temperature of the superconductingmagnet below a transition temperature of the superconducting magnet; amagnetic field controller programmed to ramp the main magnetic fieldfrom a first magnetic field strength to a second magnetic field strengthby: setting a current generated by the power supply to an initialcurrent value; activating the switch to its closed position, therebyconnecting the superconducting magnet and the power supply in theconnected circuit; adjusting the current generated by the power supply;and activating the switch to its open position when the second magneticfield strength is reached, thereby disconnecting the superconductingmagnet and the power supply from the connected circuit and placing thesuperconducting magnet in a closed circuit; a gradient system positionedabout a bore of the superconducting magnet for producing magnetic fieldgradients; a radio frequency (RF) system for transmitting RF excitationsignals and receiving MR signals; a data acquisition system connected tothe RF system to receive the MR signals from the RF system; a dataprocessing system configured to: receive the MR signals from the dataacquisition system; sort the MR signals into a plurality of MR datasets, each MR data set acquired at a distinct magnetic field strength;generate a plurality of image data sets corresponding to the pluralityof MR data sets; and combine the plurality of image data sets to providea corrected image having a reduced metal distortion; a pulse controllerconnected to the RF system, gradient system and data acquisition systemthat generates pulse sequences that include RF pulses from the RF systemand gradient pulses from the gradient system; and a computer storage forstoring the corrected image.
 10. The MR imaging system as in claim 9,further comprising: a controller connected to the RF system for settinga plurality of offset frequencies, wherein each offset frequencycomprises a central transmit frequency and a central receive frequencyset to an offset frequency value that is distinct for each offsetfrequency.
 11. The MR imaging system as in claim 10, wherein the MR datasets further comprise the MR signals received for the offsetfrequencies.
 12. The MR imaging system as in claim 9, wherein the switchis a superconducting switch.
 13. The MR imaging system as in claim 9,wherein the magnetic field controller is programmed to receive at leastone operating parameter value indicative of a present state of the MRimaging system and to select the ramp function based on the at least oneoperating parameter value and the second magnetic field strength. 14.The MR imaging system as in claim 13, further comprising a temperaturemonitor in thermal contact with the superconducting magnet so as tomeasure the temperature of the superconducting magnet, and wherein theat least one operating parameter value includes the temperature of thesuperconducting magnet.
 15. The MRI system as in claim 13, furthercomprising a magnetic field sensor proximate the superconducting magnetso as to measure the first magnetic field strength of the magnetic fieldgenerated by the superconducting magnet, and wherein the at least oneoperating parameter value includes the first magnetic field strength.16. The MRI system as in claim 13, further comprising a current monitorin electrical communication with the power supply so as to measure thecurrent generated by the power supply, and wherein the at least oneoperating parameter value includes the current generated by the powersupply.